Consolidate all doc sources

[tellyworth]

Instead of having separate sites or handbooks for the PHP docs, Gutenberg handbook etc, why don't we combine them all into one structure. They could be separate parts of the hierarchy, and retain the ability to search them separately, while having a coherent way to navigate and find everything.

For instance we have:

The function reference at https://developer.wordpress.org/reference/functions/
The common APIs handbook https://developer.wordpress.org/apis/
The block editor handbook https://developer.wordpress.org/block-editor/
The core block reference https://developer.wordpress.org/block-editor/reference-guides/core-blocks/
..and other reference guides https://developer.wordpress.org/block-editor/reference-guides/

Some of those are kind of combined already but feel like very different documents.